Strength Of Lithium-Scandium-Aluminum Alloy Resides At Nano Level

Scientists have long tried to increase the strength of materials either by adopting some metallurgical process or by inventing a new material each time. It was in 2006 that a group of scientists from #-Link-Snipped-# researching upon the same theme stumbled across a new alloy made up of Scandium-Lithium-Aluminum. The same team lead by Velimir Radmilovi? and Ulrich Dahmen have revealed how nano-particles in the Scandium-Lithium-Aluminum can disperse uniformly in the Aluminum matrix imparting it an added strength.

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When aluminum is alloyed with right proportions of scandium and lithium through a simple series of heat treatments, nanoparticle inclusions form in the aluminum matrix (dark background) whose cores, made of aluminum, scandium, and lithium (dark circles), vary in diameter and whose shells, made of aluminum and lithium (bright rings), vary in thickness. But their overall diameters are remarkably uniform. Image credit: Berkeley Lab

The scientists and their associates have used the #-Link-Snipped-# at National Center for Electron Microscopy (NCEM) specially installed for observing the ultra small objects to study the behavior of nano-particles. The acronym TEAM stands for Transmission Electron Aberration-corrected Microscope which also the scientists to understand the core shell structure of nano-precipitates. The nanoprecipitates have a peculiar tendency to retain their size over a period of time. This is different to most of the particles which change their dimensions in course of time; the smaller become smaller and larger particles expand. There by exhibiting a phenomenon called ripening or coarsening. The scientists also employed some techniques like atom-probe tomography to aid in the research.

Speaking regarding how individual elements add to strength of structures, Prof. Radmilovi? of the University of Belgrade, Serbia said that, Scandium when added in minute proportion increases the mechanical properties of Aluminum drastically while Lithium being the lightest element only after Hydrogen and Helium, lessens the overall weight of the alloy. The key to integrate the advantages offered by Lithium in the alloy lies in combining it in an L1₂ crystal structure. The structure is a face centered cubic and is the most desirable form in which Lithium should reside in. The face centered cubic structure gives the alloy an additional strength than other structures. It should be noted that Pure aluminum atoms are also found in Face centered cubic structure.

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The L12 structure is shown at lower left, with aluminum atoms in gray and scandium or lithium atoms reddish green. Image Credit: Berkeley Lab

The process of making this alloy consists of series of steps. Lithium is basically used as a catalyst which significantly lowers the temperature of the solid mix of the three metals to a temperature of 450 degree Celsius. It is heated at that temperature for about 18 hours. The prolonged heating is followed by heating of the mix to a temperature of 190 degree Celsius for 4 hours. The Lithium shells which form around the scandium rich cores are around 10.5 nanometers in thickness however nonuniform they might be. Researchers observed a pattern in the structure which they created. They found that when the core is thicker than the average the shell is thinner and vice versa. The team hence concluded that the thickness of shells and those of cores are inversely proportional to each other.

So combining the observations of the experiment and theoretical calculations team members Colin Ophus and Mark Asta stimulated a sudden burst of nucleation by modeling the effect of lithium on the solid-state precipitation of scandium.  The group was hence able to understand why and how the precipitates of the metal interacting with Aluminum are so stable and uniform.

Although the Lithium-Scandium-Aluminum is a very good alloy, still it has limited applications owing to the high costs of the rare Scandium. It will of course take further research on this topic to find an alternative for scandium or to somehow lower its cost. Do not forget to read the documentation of their work titled #-Link-Snipped-# in the journal Nature Materials.